Field of the Invention

The present invention relates to an apparatus and method for the measurement of a level and/or density profile of a material column. The present invention is particularly suited to identifying the location of one or more material interfaces in a complex multi-phase material column.

Background

Level and density measurement systems, for example for use in measuring the level and/ or density of a material within a vessel or other defined volume, are well known. It is also known to locate the boundaries between different materials in a vessel by measuring the density of the vessel contents at different locations to form a density profile of the vessel and contents in order to identify density changes which are indicative of boundary regions. For example, it has been proposed in WOOO/22387 to measure the density profile of a medium by providing an array of sources of gamma radiation to give collimated beams of said radiation and an array of detectors disposed so that the medium under study extends between the sources and the detectors. By monitoring the radiation received by the detectors, the amount of radiation absorbed by the medium from each beam can be determined and so variations in the medium density can be detected. One disadvantage of the system of WOOO/22387 is the need to employ radioactive materials which consequently imposes restrictions in use and/or health and safety issues in order to ensure safe working.

GB1524303 describes an apparatus used to determine the boundary between liquids of different densities by means of an elongated member along the length of which a series of housings is disposed with spaces between each housing into which spaces the medium under study can enter through perforations in the wall of the elongated member. Each housing comprises an ultrasonic transmitter and detector and the end of each housing acts as a reflector to reflect waves from the transmitter of the next adjacent housing back, through the medium in the space between the housings, to that next adjacent housing. One disadvantage of this type of arrangement is that the vertical resolution is limited by the need to provide a vertical space between each of the housings. The aforementioned nucleonic and ultrasonic systems are capable of detecting the interface between two different liquids of different density.

It is also known to use mechanical systems to measure the level or density of a single liquid phase. Such devices use the buoyancy force on a float or displacer weight as a means of measuring the level or density of a liquid. Examples of such mechanical systems are described in US5614672, WO2018/162541, US2015/0300864, and EP0886128. These documents describe buoyancy sensor systems comprising a single float or displacer weight as a means of measuring the level or density of a liquid. It is also known to provide systems comprising a plurality of floats at different heights within a container so that a progressive liquid level can be determined. Examples of such systems are described in GB2190500 and US4483192.

It is an aim of the present invention to provide an alternative and/or improved measurement system, apparatus and method for the measurement of a level or density profile of a material column. It is a further aim of the present invention to provide a measurement system, apparatus and method that does not require nucleonic sources.

Summary of Invention

A vertical array of pressure sensors has been considered as an alternative to the use of the nucleonic, ultrasonic or buoyancy techniques described in the background section for the measurement of a level, interface position, or density profile of a material column. However, one problem with such pressure sensors, which measure mean absolute pressure at their location, is that their accuracy varies with absolute pressure and/or varies with full scale output of the sensor. For example, a sensor capable of reading a large pressure is only accurate at the scale of that pressure. As such, a small pressure difference may go undetected. Alternatively, measurement noise and/or other inaccuracies can lead to an erroneous indication of a pressure difference when there isn't one. Pressure sensors can be tailored for operation over a specific pressure range. However, a material column may comprise materials at very different pressures. For example, a material column may comprise a dense liquid-solid suspension, a liquid, a foam, and a gas moving upwards through the column. The height of the interfaces between material phases may change significantly during operation of a process. As such, in many cases it is not possible to deploy a pre-determined pattern of different types of pressure sensor through a material column to accurately measure the absolute pressure of different materials, having different densities, distributed throughout the material column. In this regard, while there may be a desired location for interfaces between the phases for optimal performance of a process, which could be mapped using different types of pressure sensor, one goal of monitoring a level or density profile of a material column is to detect when material levels or interfaces between materials are not where they are intended to be. That is, the sensor system is intended to detect a defect in a process such that the defect can be corrected. As such, designing a pattern of pressure sensors which is optimized for accuracy during normal operation of a process will not necessarily provide an accurate measurement system for when a process is not functioning in its intended manner.

In addition to the above, absolute measurements may be adversely affected during operation. Absolute measurement systems may deteriorate in their accuracy due, for example, to fouling of sensors or local anomalies.

In light of the above, the present specification proposes instead to use an array of differential pressure sensors to measure a level and/or density profile of a material column and/or interface locations within a material column. The accuracy of a differential pressure measurement is independent of the mean absolute pressure, allowing differences in pressure to be accurately measured regardless of the absolute pressure. Furthermore, differential pressure sensors take a local, relative measurement which can, in certain applications, be more robust to changes in functionality over time in operation when compared to an absolute measurement, since local variations can cause a similar drift in absolute measured parameter values while a local differential measurement can remain reasonably stable and reliable. Further still, a differential pressure measurement can be used to extract local material density information which is desirable for profiling a material column, identifying different types and locations of materials, and locating the interfaces between different materials in a column.

Despite the potential advantages of a differential pressure measurement as outlined above, a problem exists with implementing such an approach for profiling a material column. Differential pressure gauges generally comprise a thin measurement diaphragm or membrane which is deflected by fluids on either side of the diaphragm having different pressures. As the measurement diaphragm of a differential pressure gauge is generally thin, if such a sensor is simply placed in a fluid column the pressures either side of the diaphragm will be almost the same, unless the diaphragm is located precisely at an interface between different materials of different density in the fluid column, and so little or no pressure difference will be recorded.

As such, the present specification proposes an array of differential pressure gauges in which each differential pressure gauge comprises two spaced apart actuators which are coupled together such that the pressure difference between the one or more material phases in the material column at the two spaced apart actuators can be measured. Such an array can be configured to extend down through a material column such that each differential pressure gauge is located at a different height within the fluid column. Furthermore, each differential pressure gauge is orientated such that the spaced apart actuators are located at different heights within the material column with a coupling which can comprise a component which is deflected as a result of the pressure differential between the two spaced apart actuators at different heights. This deflection can be measured by a sensor and used to detect and measure the local pressure differential between the two spaced apart actuators due to the difference in height of the two actuators within the material column. Such an array can therefore provide a profile of local pressure differences through the material column which can be used to calculate a density profile, identifying different types and locations of materials, and locate interfaces between different materials of different density within the material column. The present specification thus provides an apparatus for determining the identity, composition, location, density and/or level of material phases in a material column and/or the location of an interface between two material phases in the material column. The apparatus comprises: an array of differential pressure gauges, the array being configured to be at least partially submerged within one or more material phases in the material column; wherein each differential pressure gauge comprises two spaced apart actuators which are coupled together via a coupling such that a pressure difference between the one or more material phases in the material column at the two spaced apart actuators can be directly measured by each of the differential pressure gauges when the array of differential pressure gauges is at least partially submerged within the one or more material phases in the material column, the apparatus being configured to process the differential pressure measurements from each of the differential pressure gauges to determine the identity, composition, location, density and/or level of the one or more material phases and/or the location of an interface between two material phases within the material column.

It should be noted that the two spaced apart coupled actuators enable a pressure difference to be directly measured between one or more material phases in the material column at the two spaced apart coupled actuators. This differs from configurations which take differential pressure measurements at individual locations relative to a reference fluid/gas. The present configuration does not require such reference fluid/gas. The present configuration also differs from systems which take individual pressure measurements and then calculate a pressure difference. Furthermore, the present configuration takes direct differential pressure measurements when submerged within the material phases in the column. No external couplings are required.

The actuators of each differential pressure gauge in the array can be spaced apart by a sufficient distance so as to reliably measure a pressure differential due to a differential in height of a material column between the two actuators. Typically, this spacing or height differential will be: at least 1 cm, 2 cm, 3 cm, 4 cm, or 5 cm; no more than 50 cm, 40 cm, 30 cm, 20 cm, 15 cm, or 10 cm; or within a range defined be any of the aforementioned lower and upper limits. This will enable pressure differences to be measured within an individual phase of material due to a difference in height of the measurement points corresponding to the spaced apart actuators. In this regard, the static differential pressure between two vertically separated points in a fluid column of a single phase of material is given by the formula:

DR = pxgx dh where DR is the differential pressure, p\s the density of the material, g is acceleration due to gravity, and dh is the vertical separation between measurement points. For an unknown material, by measuring DR, and knowing g and dh, the density of the material pc an be calculated. In this regard, it should be noted that in practice the calculation is more complicated as the effect of the components between the actuators within the differential pressure gauge should be accounted for. The more detailed calculation is explained in the detailed description of this specification.

The density of different materials (e.g. fluids) in complex multi-layered fluid systems such as oil/water separator vessels in refineries can thus be measured. Of course, if an interface between two different materials of different density is located between the two actuators of one of the differential pressure gauges in the array then a larger pressure differential will be measured indicative of the interface location at the location of the differential pressure gauge. However, even if the interface is located between differential pressure gauges in the array, the location of the interface will be evident due to the different densities of the materials detected by the two differential pressure gauges either side of the interface.

The number and spacing of the differential pressure gauges in the array can be varied according to end use requirements. A larger number of more closely spaced differential pressure gauges will allow the location of an interface to be more reliably determined with greater precision. Typically, the array will comprise: at least 3, 5, 10, 20, or 50 differential pressure gauges; no more than 500, 300, or 100 differential pressure gauges; or a number of differential pressure gauges which lies in a range defined by any of the aforementioned lower and upper limits.

The array of differential pressure gauges will typically be configured as an elongate (e.g. linear) array which can be introduced into a material column in a vertical orientation so as to provide an array of differential pressure gauges which extends down (or up) through the material column, with each differential pressure gauge located at a different height within the material column. Each differential pressure gauge in such an array can be oriented so that the spaced apart actuators are located at different heights within the material column. This arrangement enables differential pressure measurements at different heights through a material column. Other configurations may also be implemented. For example, pressure gauges may be located at different lateral locations to give information about density variations across a material column at one or more height locations. Pressure gauges may overlap in a height/depth direction to increase the precision with which interfaces can be located. For example, the pressure gauges can be mounted on an elongate support in a helical configuration which enables the gauges to be mounted in an overlapping fashion with the array extending in an elongate, helical manner down through the material column.

The coupling between the spaced apart actuators may be provided by a pressure transfer media (e.g. a fluid of known density) disposed between the actuators. Alternatively, or additionally, a mechanical coupling can be provided between the two actuators. The mechanical coupling can be configured in different ways. For example, the mechanical coupling may be provided by rigid mechanical coupling such as a rod between the two actuators. Alternatively, each actuator may be coupled via a spring to transfer a force resulting from external pressure on the actuators to a central measurement/sensor unit.

The actuators may be provided by flexible diaphragms. Alternatively, the actuators may be rigid actuators, mounted to be sealed against fluid ingress but movable in a piston type configuration within a housing when subjected to external pressure.

For example, according to one configuration, each actuator may be formed of a diaphragm with a pressure transfer media disposed between the diaphragms. A third, measurement diaphragm may be located within the pressure transfer media which is deflected as a result of the pressure differential between the two actuators. The defection of the measurement diaphragm can then be used to detect and measure the pressure differential between the two spaced apart actuators. According to an alternative configuration, each actuator may be formed of a rigid piston with a rod connecting the pistons which is deflected as a result of the pressure difference between the pistons. The defection of the rod can then be used to detect and measure the pressure differential between the two actuators. The piston and rod configuration can be sprung or otherwise biased to an equilibrium position when no differential pressure is applied.

While the aforementioned configurations comprise an array of differential pressure gauges, in an alternative configuration the apparatus can comprise a single differential pressure gauge which is mounted to be movable through a material column to profile the material column. In this respect, there is provided an apparatus for determining the identity, composition, location and/or level of one or more material phases in a material column and/or the location of an interface between two material phases in the material column, the apparatus comprising: a differential pressure gauge; and a drive mechanism to which the differential pressure gauge is mounted, the drive mechanism configured to move the differential pressure gauge through a material column comprising one or more material phases; wherein the differential pressure gauge comprises two spaced apart actuators which are coupled together via a coupling such that a pressure difference between the two spaced apart actuators can be measured when the differential pressure gauge is moved through the material column by the drive mechanism, the apparatus being configured to take differential pressure measurements at a plurality of depths through the material column and process the differential pressure measurements to determine the identity, composition, location, and/or level of the one or more material phases and/or the location of an interface between two material phases within the material column.

In yet another alternative configuration the apparatus can comprise a single differential pressure gauge which is fixedly mounted within a material column. In this respect, there is provided an apparatus for determining the identity, composition, location and/or level of one or more material phases in a material column and/or the location of an interface between two material phases in the material column, the apparatus comprising: a differential pressure gauge; and a mounting for the differential pressure gauge to mount the differential pressure gauge within a material column comprising one or more material phases; wherein the differential pressure gauge comprises two spaced apart actuators which are coupled together via a coupling such that a pressure difference between the two spaced apart actuators can be measured when the differential pressure gauge is mounted within the material column, the apparatus being configured to take one or more differential pressure measurements within the material column and process the one or more differential pressure measurements to determine the identity, composition, location, and/or level of the one or more material phases and/or the location of an interface between two material phases within the material column.

The apparatus configurations as described herein are intended to be used for determining the identity, composition, location and/or level of one or more material phases in a material column and/or the location of an interface between two material phases in a material column. Configurations can determine one, more, or all of these parameters. The methodology comprises: introducing the apparatus into the material column; measuring a pressure difference at one or more depths within the material column; and processing the one or more pressure difference measurements to determine the identity, composition, location, and/or level of the one or more material phases and/or the location of an interface between two material phases within the material column.

Description of the Drawings

Embodiments of the present invention will now be described, by way of example only, with reference to the accompanying drawings, in which:

Figure 1 shows a schematic of a conventional differential pressure gauge;

Figure 2 shows the differential pressure gauge of Figure 1 within a fluid column illustrating that such a differential pressure gauge is not well suited to measuring pressure differences in a fluid column; Figure 3 shows a schematic of a different type of differential pressure gauge which comprises two spaced apart actuators enabling measurement of pressure difference between the two spaced apart actuators in a fluid column;

Figure 4 shows a variant of the arrangement shown in Figure 3 in which the actuators are coupled by a rigid rod rather than via a pressure transfer medium as in Figure 3;

Figure 5 shows a schematic of a differential pressure gauge of the type illustrated in Figure 3 or Figure 4 to illustrate how density of the fluid surrounding the gauge can be calculated;

Figure 6 shows a schematic of an apparatus comprising a linear array of differential pressure gauges of the type illustrated in Figure 3 or Figure 4; and

Figure 7 shows a variant of the arrangement shown in Figure 6 in which the differential pressure gauges are disposed within a tubing which has inlets for enabling surrounding fluid to act upon the actuators of the differential pressure gauges.

Detailed Description

As described in the background section, the present specification proposes to use an array of differential pressure gauges for determining the identity, composition, location and/or level of one or more material phases in a material column and/or the location of an interface between two material phases in a material column.

A conventional differential pressure gauge may, for example, take the form of an instrumented membrane, diaphragm or sprung piston separating two fluid filled chambers with substantially rigid walls. If the fluid(s) either side of the measurement diaphragm is/are at different pressures, they will exert an unbalanced force on the diaphragm. The resulting deflection can be measured by a suitable sensor, for example, using a mechanical measurement (e.g. a strain gauge), an electrical measurement (e.g. a capacitance measurement), or an optical measurement. The sensor output can be calibrated to allow differential pressure to be measured.

Such a conventional differential pressure gauge is illustrated schematically in Figure 1. The differential pressure gauge comprises a rigid housing 2 with two chambers 4, 6 having respective fluid inlets 8, 10 for fluid at two different pressures Pi, P . The two chambers 4, 6 are separated by a diaphragm 12 which is subjected to fluid pressure on either side of the diaphragm. For fluids at different pressures Pi, P the diaphragm will be defected towards the lower pressure side. This defection of the diaphragm 12 can be detected by a sensor 14 coupled to the diaphragm 12 and used to measure the pressure difference DR = PI - P2 when suitably calibrated.

Figure 2 shows the differential pressure gauge of Figure 1 within a container 20 filled with a fluid column 22. As described in the summary section, the static differential pressure between two vertically separated points in a fluid column of a single phase of material is given by the formula:

DR = pxgx dh where DR is the differential pressure, p\s the density of the material, g is acceleration due to gravity, and dh is the vertical separation between measurement points. In the arrangement shown in Figure 2, the pressure gauge has a single actuator in the form of a single diaphragm 12, and dh will correspond merely to the thickness of the diaphragm. As the measurement diaphragm of a differential pressure sensor is generally thin, such that it can readily be defected by a pressure difference, if such a differential pressure gauge is simply placed in a fluid column the pressures either side of the diaphragm will be almost the same, and so little or no pressure difference will be recorded. Furthermore, for fluid columns comprising different materials of different density, the materials entering the chambers 4, 6 may become trapped within the chambers leading to erroneous measurements when the levels of materials subsequently change within the container 20. Such a differential pressure gauge is therefore not well suited to measuring pressure differences in a fluid column.

Figure 3 shows a schematic of a different type of differential pressure gauge within a container 20 filled with a fluid column 22. The differential pressure gauge comprises a rigid housing 2 with two chambers 34, 36 separated by a diaphragm 12. In this respect the differential pressure gauge is similar to that previously described and illustrated in Figures 1 and 2. However, chambers 34, 36 do not have inlets to allow external fluid to flow into the chambers and impart a pressure directly on the diaphragm 12 as in the previous configuration. Rather, the chambers 34, 36 are capped / sealed by actuator diaphragms 30, 32 with a pressure transfer medium in the form of a fluid of known density within the chambers 34, 36. In this configuration, fluid external to the gauge imparts a pressure Pi, P2 on the actuator diaphragms 30, 32 and this external pressure on each actuator diaphragms 30, 32 is transmitted through the pressure transfer medium in chambers 34, 36 to the measurement diaphragm. The measurement diaphragm 12 is therefore subjected to fluid pressure on either side of the diaphragm as before. However, in this case the pressures on either side of the diaphragm 12 equate to the pressures imparted on the spaced apart actuator diaphragms 30, 32. That is, dh equates to the distance between the actuator diaphragms 30, 32 rather than merely the thickness of the measurement diaphragm 12. As such, a pressure difference due to the difference in height between the actuator diaphragms 30, 32 deflects the measurement diaphragm 12 and this deflection can be detected by a sensor 14 coupled to the diaphragm 12 and used to measure the pressure difference DR = PI - P2 when suitably calibrated. Since DR = px g x dh, and since DR has been measured and g and dh are known, then the density p of the fluid 22 can be calculated. In this regard, it should be noted that in practice the calculation is more complicated as the effect of the components between the actuators within the differential pressure gauge should be accounted for. The more detailed calculation is described later in this specification with reference to Figure 5. In addition to providing a larger dh value to enable a pressure differential to be measured at spaced apart heights in the fluid column, this type of configuration also has the advantage of sealing chambers 4, 6 such that material from the material column does not get trapped adjacent to the measurement diaphragm 12. As such, this type of configuration is much better suited to measuring pressure differences in a material/fluid column to profile the column.

Figure 4 shows a variant of the differential pressure gauge shown in Figure 3 disposed in a container 20 filled with a fluid 22. The differential pressure gauge comprises a rigid housing 2 capped or sealed by two actuators 40, 42. The actuators 40, 42 can be flexible diaphragms as previously described or can be rigid disks which are sealed against the interior of the housing 2 to prevent fluid ingress but movable along the housing. In the latter configuration, the housing 2 should extend beyond the actuators 40, 42 to enable movement of the actuators in an upwards and downwards direction in a piston type configuration without exiting the housing 2. A rod 44 connects the actuators 40, 42 such that the rod will be deflected as a result of a pressure difference between the actuators 40, 42. The defection of the rod can then be used to detect and measure the pressure differential between the two actuators using a sensor 48, 14. For example, the rod may act on a spring and/or diaphragm to deflect the spring/diaphragm, with this defection being used to measure the pressure differential. The spring/diaphragm also serves to hold the rod within the housing 2. For example, the piston and rod configuration can be sprung to be biased to an equilibrium position when no differential pressure is applied. In the illustrated configuration, a single chamber 46 is provided. This may be filled with a fluid of known density. In an alternative configuration, the chamber can be split into two by a measurement diaphragm. In this case, the configuration is similar to the configuration previously described but rigid actuators can be used instead of flexible actuator diaphragms.

It will be appreciated that the variants illustrated in Figures 3 and 4 are only examples of differential pressure sensors with spaced apart actuators and that alternatives or combined configurations are also envisaged. The main requirement is that the spaced actuators are coupled such that a pressure difference between the spaced apart actuators can be measured in a material column. Such configurations are suitable for measuring a pressure difference in a material/fluid column and calculate a density of the material/fluid in the column at a particular location within the column.

Figure 5 shows a schematic of a differential pressure gauge of the type illustrated in Figure 3 or Figure 4 to illustrate in more detail how density of the fluid surrounding the gauge can be calculated. As previously described, the differential pressure gauge can be constructed with chambers either side of the measurement diaphragm filled with fluid of known density (pressure transfer media) and the ports capped with a diaphragm or other movable / flexible seal. The fluid pressure at the vertical level of the flexible caps is transferred to the measurement diaphragm. The instrumented diaphragm may also or alternatively be acted upon by remote diaphragms with solid pushrods or similar arrangements to transfer the force generated by the external fluid. When the sensor assembly is placed in a fluid column with one capped port vertically above another, a sufficiently large differential pressure is imparted and can be measured. External mean density may then be calculated or derived directly through calibration. In this regard, if the density is to be calculated, rather than derived directly through calibration, the effect of the internal pressure transfer medium should be accounted for in the calculation as set out below.

In Figure 5 components are labelled as follows:

1. Process fluid, or fluid of interest with density p(o)

2. Port cap or diaphragm (1)

3. Density sensor body

4. Pressure Transfer Fluid (1) with density p(il)

5. Measurement Diaphragm

6. Pressure Transfer Fluid (2) with density p(i2)

7. Port cap or diaphragm (2)

In Figure 5, parameters for calculating the density of the external fluid are as follows:

Pol: External pressure marginally outside of Port cap or diaphragm (1)

Po2: External pressure marginally outside of Port cap or diaphragm (2)

Pil: Internal pressure marginally inside of Port cap or diaphragm (1)

Pi2: Internal pressure in pressure transfer fluid (1) marginally above measurement diaphragm Pi3: Internal pressure in pressure transfer fluid (2) marginally below measurement diaphragm Pi4: Internal pressure marginally outside of Port cap or diaphragm (2)

HI: Vertical separation from measurement diaphragm to port cap or diaphragm (1)

H2: Vertical separation from measurement diaphragm to port cap or diaphragm (2) g: acceleration due to gravity

Pol is externally determined. As the system is a differential system its absolute magnitude is unimportant for calculation purposes.

1. Pol = Pol

2. Po2 = Pol + p(o)*g*(Hl+H2)

As port cap or diaphragm (1) is flexible and vertical separation is small: 3. Pil = Pol

4. Pi2 = Pol + p(il)*g*Hl

5. Pi3 = Po2 - p(i2) * g * H2 = Pol + p(o)*g*(Hl+H2) - p(i2) * g * H2 As Port cap or diaphragm (1) is flexible and vertical separation is small:

6. Pi4 = Po2

Differential pressure DR across measurement diaphragm (measured by density sensor):

7. DR = RΪ2-RΪ3 = (Pol + p(il)*g*Hl)-(Pol + p(o)*g*(Hl+H2) - p(i2) * g * H2)

8. - DR = - p(il)*g*Hl+ p(o)*g*(Hl+H2) - p(i2) * g * H2

9. p(o) = (p(il)*g*Hl + p(i2) * g * H2 - DR) / (g*(Hl+H2))

All terms in this equation are known, so p(o) may be calculated.

In the case that the fluid transfer media and the port separations are equal, the equation simplifies to

10. p(o) = (2* p(i)*g*H - DR) / (2*g*H))

In the case that there are multiple external fluids between the port caps, p(o) will be the average density. Information about the level of the interface may be inferred from the densities of the media above and below.

The pressure transfer media may be a grease, a liquid, a gas, a gel, a flexible solid, or a combination of these media. It must have the capability of transmitting pressure from the process fluid to the measurement diaphragm or alternate measurement unit. Advantageously, due to its compressibility, if gas is used to make up some or all of the pressure transfer fluid, by limiting port cap travel/deflection excessive system pressures can be prevented from reaching and potentially damaging the measurement diaphragm. One or more pressure relief valves may be used to provide a protective function with other pressure transfer media.

The ports may, for example, take the form of a hole or spigot forming an attachment point for a tube or pipe or may simply be an opening into the diaphragm chamber. The port caps must be capable of transferring pressure from the process fluid to the pressure transfer fluid and may, for example, take the form of a flexible diaphragm, piston or gel plug. The port caps may also take the form of a close-ended flexible tube or other closed geometry. In the case of a grease, gel or flexible solid pressure transfer media, the exposed outermost surface of the media may form the port caps. Advantageously, the port caps may also serve to protect the differential pressure sensor from contamination, build-up, blockage, electrical damage or error, or attack by process fluid(s).

The vertical separation between the sensor port caps may be chosen, for example, to give a maximum differential pressure in line with the sensor's capability, therefore maximising resolution. Similarly, a pressure sensor may be designed or selected to be optimised for sensing at a specific vertical separation for a given density range. The vertical separation between sensor port caps may be tuned to suit a specific application, for example, by rotating a sensor whilst keeping the ports at a fixed distance.

An instrument may incorporate additional sensors such as temperature, absolute pressure or accelerometers. A temperature sensor may allow correction for effects such as pressure transfer media expansion or thermal effects on the electrical characteristics of the sensor. An accelerometer may be used to determine the angle the sensor is placed at and therefore the vertical separation or the port caps.

A single static differential pressure gauge positioned at a single location within a fluid column is not suitable for determining a vertical level/density profile of a complex multi-layered fluid column including at least two different liquids or a fluid column including at least one liquid with a varying solid content along the fluid column. In this case, two possible configuration types are envisaged as described below.

In one configuration type, an apparatus is provided in which a single differential pressure gauge as previously described. The differential pressure gauge can be configured to be fixedly mounted within a material column. This may be, for example, to determine if a fluid level reaches a threshold level at which some action is required, e.g. a change in fluid flow settings or an alarm mechanism or automated shut off. Alternatively, the differential pressure gauge can be mounted to be movable along the fluid column so as to take differential pressure measurements at different depth to profile the fluid column. For example, the differential pressure gauge can be mounted on the end of a cable so that it can be lowered down the fluid column and pulled back up the fluid column, i.e. a winch configuration. Alternatively, the differential pressure gauge can be mounted to an elongate, vertically orientated track such that it can be moved down the fluid column. Alternatively still, the differential pressure gauge may be mounted on a remotely operated vehicle or submersible. The apparatus can be configured to take differential pressure measurements at a plurality of depths and process the measurement results to generate a profile, e.g. a density profile, of the fluid column and/or to identify different types of material in the fluid column and/or to locate interfaces between different materials in the fluid column.

In another configuration, an array of differential pressure gauges is provided. Figure 6 shows a schematic of an apparatus 50 comprising an elongate array of differential pressure gauges 54 of the type illustrated in Figure 3 and Figure 4. The apparatus comprises an elongate housing 52 to which the array of differential pressure gauges 54 are mounted. The apparatus comprises a mounting 62 such that the elongate array can be mounted in a container 60, such as a separator unit, comprising different materials Mi, M ₂ , M ₃ , M ₄ having different densities and located at different heights within the container 60. For example, the material layers may comprise one or more of a solid suspension or sludge layer, an aqueous layer, an emulsion, an oil layer, a foam layer, and/or a gas layer. The materials may not necessarily be liquids. For example, a granular solid material can be fluidized to behave as a fluid with a pressure proportional to the density of the granular solid. Alternatively, the material may be a solid-liquid suspension with a varying solid content.

The differential pressure gauges 54 are connected by wiring 56 to a control unit 58 which can be located outside the container 60. Alternatively, wireless communication between the differential pressure gauges and the control unit can be utilized for measurement control and data acquisition.

The apparatus is thus configured to take differential pressure measurements from each of the differential pressure gauges to determine the identity, composition, location, and/or level of the one or more material phases and/or the location of an interface between two material phases within the material column. Figure 7 shows a variant of the arrangement shown in Figure 6 in which the differential pressure gauges 68 are disposed within a tubing 64 which has inlets 66 for enabling surrounding fluid to act upon the actuators of the differential pressure gauges 68. As in the configuration of Figure 6, the differential pressure gauges 68 are connected to a control unit 70 for measurement control and data acquisition. This configuration has the advantage of protecting the differential pressure gauges 68 within the tubing 64 to prevent the gauges becoming damaged by mechanical impact during transportation, installation, and use of the apparatus.

Depending on end applications and desired functionality, various embodiments of the apparatus are envisaged. For example, the differential pressure gauges could be tailored for different functions such as critical level signalling, trip systems, combinations of level and density information for trays in distillation columns, etc.

In addition to the provision of a vertical array of differential pressure gauges, the differential pressure gauges can also be spaced part from one another radially around an elongate vertical support member. This configuration can be used to reduce the vertical spacing between gauges and therefore increase the precision with which an interface between two fluid layers can be measured. The configuration can also give information about horizontal variations in density in addition to vertical variations in density.

A data processing unit can be configured to use the density profile to indicate the vertical level of each fluid layer within the multi-layered fluid column. The apparatus may also comprise one or more temperature sensors. Temperature corrections can be applied when processing the measurement data to generate a density profile. Additionally, or alternatively, density information can be combined with pressure and/or temperature information to give an indication of operating conditions within a vessel.

The apparatus as described herein can be used in complex multi-layered fluid systems such as oil/water separator vessels in refineries. As such, also provided is a system comprising an oil/water separator vessel and an apparatus as described herein disposed within the oil/water separator vessel. The rates of flow to the inlet and/or from the outlets of such a vessel may be controlled in response to the monitored levels of the oil/water and/or gas/liquid boundaries.

Normally, the number of phases present in a vessel and their approximate composition is known and this information may be used to interpret the information gained from an apparatus of the present specification. For example, the phases in an oil separator would be expected to include oil, water, gas, an oil/water emulsion, a foam, and a heavy phase at the bottom of the vessel containing sand or other solids. It is possible to pre-configure the apparatus as described herein to identify the materials and their locations from the differential pressure measurement data. It is also possible to pre-configure the apparatus to use density measurements to monitor acid concentration in alkylation reactions.

Applications of the apparatus and methods as described herein can include: chemical/materials processing; oil/water separators; underwater grouting; paper/pulp processing; mining applications; monitoring of distillation columns; and monitoring of storage tanks.

While this invention has been particularly shown and described with reference to certain examples, it will be understood to those skilled in the art that various changes in form and detail may be made without departing from the scope of the invention as defined by the appended claims.